The present invention relates to turbomachines, and more particularly to the casings of turbomachines.
In the present context, the term “turbomachine” refers to any machine in which a transfer of energy can take place between a fluid flow and at least one set of blades, such as, for example, a compressor, a pump, a turbine, or also a combination of at least two of them. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of fluid through the turbomachine.
Such a turbomachine may comprise a plurality of stages, each stage usually comprising two sets of blades, i.e. a movable set of blades and a stationary set of guide blades (or “vanes”). Each set of blades comprises a plurality of blades that are offset from one another in a lateral direction. Typically, these blades are arranged radially about a central axis A. Thus, such a set of blades forms a rotor, when it is a movable set of blades, or a stator, when it is a set of guide vanes. The proximal end of each blade relative to the central axis A is usually referred to as the blade “root”, whereas the distal end is usually referred to as the blade “tip”. The distance between the blade root and the blade tip is known as the blade “height”. Between the blade root and the blade tip, the blade is made up of a stack of aerodynamic profiles that are substantially perpendicular to a radial axis Y. In this context, the term “substantially perpendicular” means that the plane of each profile may present an angle relative to the radial axis Y that is close to 90°, e.g. in the range 60° to 120°.
In a turbomachine, such a rotor is usually surrounded by a casing. During operation of the turbomachine, aeroelastic instabilities may give rise to rotating waves of distortion in an annular element of the casing, and consequently, it may even give rise to cracks in the material. In order to reduce that sensitivity of casing structures to aeroelastic instabilities, it has been proposed in particular to make such an annular casing element substantially non-axisymmetric, in such a manner as to prevent local natural modes of vibration from combining into a single rotating wave. However, such a solution presents the drawback of making the design and fabrication of casing elements considerably more complicated. For damping the vibration of rotor blades or airfoils, it is also known to wedge dampers underneath the platforms of the blades, across adjacent blade platforms. When the rotor is rotating, centrifugal forces urge each damper against the bottom surfaces of two adjacent blades, and the friction between the damper and said two surfaces contributes to dissipating the vibrations of the two adjacent blades. By way of example, dampers of that type are disclosed in French patent FR 2 923 557 B1. However, since the operation of dampers of that type depends on centrifugal forces, they cannot be applied to stationary parts such as the stator or the casing.